The field of the present invention is keyboards and, more particularly, membrane keyboards.
The use of membrane keyboards is considered advantageous in applications where a sealed matrix is desired, construction is to be simplified, and space is to be conserved. These keyboards are typically thin, utilizing switches which have only a short actuating excursion and are of the normally open type. In order to prevent the introduction of contaminants, and to facilitate the identification of keys, it has been known to provide these keyboards with a flexible plastic cover known as a coverlay.
A disadvantage of plastic coverlays is that plastic suffers undesirable wear from abrasion and from flexing, leading to permanent deformation of the coverlay and obliteration of the key indicia. Another disadvantage of plastic coverlays is that these coverlays may be subject to the destructive effects of chemicals such as solvents and caustic substances. This disadvantage is particularly pronounced where the keyboard is intended for use in a laboratory or industrial environment.
Biological and chemical contamination are considered to be hazardous in the presence of conventional switches which have crevices and minute openings. A membrane coverlay would facilitate decontamination, but many plastics are degraded by chemicals used in decontamination.
A further disadvantage is that plastic coverlays suffer degradation in the presence of heat and ultraviolet light. Radio-frequency radiation is also known to pass through plastic keyboards and interfere with electronic functions of the respective device or other devices. Consequently, keyboard devices are frequently provided with radio-frequency shielding.
It has been known to address some of the disadvantages discussed above with the provision of a metal keyboard cover having hinged key actuators. Such a device is shown in the patent issued to Hashimoto, et al., U.S. Pat. No. 4,338,502, issued July 6, 1982. In this device, the key actuators are made movable by cutting a slot through the cover and partially surrounding each keyboard actuator. This pierced keyboard cover suffers from the disadvantages of allowing the entry of degrading factors such as light, dust, and chemicals into the keyboard and possibly into the mechanism itself. Additionally, the thickness of such a cover is restricted in that the hinge's angle of flex must, under normal pressure, be sufficient to permit the amount of travel required to actuate the electrical contacts lying beneath each key site.
Another device has been discussed in the patent issued to Komaki, U.S. Pat. No. 4,249,054, issued Feb. 3, 1981, which calls for a metal calculator keyboard cover to be made extremely thin contiguous with each key site. Although the metal keyboard cover taught in Komaki is substantially continuous, the thinner areas of the key sites are taught to be in the region of 0.025-0.03 millimeters. This extremely thin dimension is disadvantageous in that a keyboard of that design will not exhibit the desired wear characteristics and may be subject to permanent deformation. These disadvantages are aggravated where the keyboard is relatively large or where it is used in a hostile environment.
A further disadvantage of membrane keyboards of known type is that these keyboards are composed entirely of normally open, single-throw switches. Although many designs of normally closed switches exist, they are too bulky to satisfy the space requirements of a membrane keyboard. Where the circuitry with which the keyboard is to be associated requires the use of a normally closed switch signal, an expedient such as a relay or solid-state inverter must be used. Even where such devices are used, contact bounce not present in normally closed switches may be introduced.
Similarly, multiple throw switches exist in many forms, but all require excessive volume or excursion for efficient use in a membrane keyboard. Conventional switches also carry a risk of explosion in combustible environments unless carefully sealed against the exposure of internal sparking.